Aurora r6g instances are 1 vCPU per 8GB memory at $0.12978 per vCPU hr. The equivalent on AlloyDB is $0.15568
Aurora x2g instances are 1 vCPU per 16GB memory at $0.1885 per vCPU hr. The equivalent on AlloyDB is $0.24528
Given the additional memory/cache characteristics and the 2x performance claims, I'm guessing you'll want to go memory heavy, so it _might_ not be terrible to say that a 2 vCPU/16GB Aurora instance ($0.26/hr) may perform similarly to a 1 vCPU/16GB AlloyDB instance ($0.24528/hr).
Storage is cheaper on AlloyDB and data transfer likely is as well.